Natural resources present in subterranean formations or zones, such as gas, petroleum, and water are usually recovered by making a borehole so as to reach a subterranean formation while circulating a drilling fluid in the borehole. A fluid for subterranean formation processing such as a fracturing fluid, mud water, a cementing fluid, a well control fluid, a well kill fluid, an acid fracturing fluid, an acid diverting fluid, a stimulation fluid, a sand control fluid, a completion fluid, a wellbore consolidation fluid, a remediation treatment fluid, a spacer fluid, a drilling fluid, a frac-packing fluid, a water conformance fluid, or a gravel packing fluid is used for the recovery of petroleum or gas.
The subterranean formation processing fluid is supplemented with additives such as a lost circulation preventing agent, a density agent, a dispersant, a surfactant, a viscosity adjuster, and a thickener. For example, the mud water is prepared by dispersing bentonite, mica, hydrated lime, carboxymethylcellulose, silicone resin, and the like in a liquid carrier such as water or an organic solvent for the purpose of, for example, reducing the friction between a drill and side walls, cooling a bit, taking crushed rocks out, preventing exfiltration or lost circulation during drilling operation, or preventing the collapse of side walls formed by excavation.
In the recent production of petroleum or natural gas, very deep well drilling has been practiced. Thus, there has been a demand for a material for subterranean formation processing fluids that resists high temperatures. A polymer containing a prepolymer having no LCST (lower critical solution temperature) and a prepolymer having LCST (lower critical solution temperature) has been reported as a composition whose viscosity is increased with elevation in temperature (Patent Literature 1).
Meanwhile, fine cellulose fibers (Patent Literature 2) and cellulose nanocrystals produced by an acid hydrolysis method (Patent Literature 3) are known as components of thickeners for such purposes. Also, a composition for subterranean formation processing comprising nanocrystalline cellulose has been proposed (Patent Literature 4). In recent years, a revolutionary method for producing nanosized ultrafine cellulose fibers with cellulose crystallinity maintained has been further developed (Non Patent Literature 1). In this method, an aldehyde group or a carboxy group is introduced to the surface of cellulose fibers, followed by machine processing to generate fine cellulose fibers. Unlike cellulose nanocrystals, the ultrafine cellulose fibers obtained by this method retain a non-crystalline region in addition to a crystalline region. Therefore, long fibers are obtained and can make a gel having a high viscosity in water. Use of the ultrafine cellulose fibers produced by this method in cosmetic thickeners or drilling thickeners has been proposed (Patent Literatures 5 and 6).
For the production of ultrafine cellulose fibers, the ultrafine cellulose fibers are often produced at a solid concentration of 5% by mass or smaller and are provided as a low-concentration gel. A method for increasing the solid concentration has been proposed because of reduction in transportation cost or poor handleability attributed to a large amount of water (Patent Literatures 7, 8, and 9).
The ultrafine cellulose fibers are also used as a highly viscous thickener. For use in cosmetic thickeners, drilling thickeners, or the like, products are often supplemented with salts. Thus, the ultrafine cellulose fibers have been required to exert viscosity even in the presence of a salt.